1. Field of the Invention
This invention relates generally to a fuel cell system that includes a procedure for using air for purging hydrogen from a fuel cell stack at system shut-down and, more particularly, to a fuel cell system that includes a procedure for using air for purging hydrogen at system shut-down, where the procedure includes delaying the air purge until the temperature of the fuel cell stack is reduced below a predetermined temperature so as to reduce cell deterioration.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode, typically by a catalyst, to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons, typically by a catalyst, in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorinated acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred stacked fuel cells. The fuel cell stack receives a cathode input gas as a flow of air, typically forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
When a fuel cell system is shut down, unused hydrogen gas remains in the anode side of the fuel cell stack. This hydrogen is able to diffuse through or cross over the membrane and react with the oxygen within the catalyst layer on the cathode side. As the hydrogen diffuses out of the anode side to the cathode side, the total pressure within the anode side is reduced below the ambient pressure. This pressure differential sucks air from the ambient into the anode side of the stack. When the air enters the anode side of the stack it generates an air/hydrogen front that creates a short circuit in the anode side, resulting in a lateral flow of hydrogen ions from the hydrogen flooded portion of the anode side to the air-flooded portion of the anode side. This high ion current through the high lateral ionic resistance of the membrane produces a significant (˜0.5 V) lateral potential drop in the membrane. This produces a local high potential between the cathode opposite the air-filled portion of the anode and adjacent to the electrolyte that drives rapid carbon corrosion, and causes the carbon layer to get thinner. This decreases the support for the catalyst particles, which decreases the performance of the fuel cell.
It is known in the art to purge the hydrogen out of the anode side of the fuel cell stack at system shut-down by forcing air into the anode side at high pressure from the cathode input. The air purge creates the air/hydrogen front that causes the cathode carbon corrosion, as discussed above. Thus, it is desirable to reduce the air/hydrogen front residence time to as short as possible, where the front residence time is defined as the anode flow channel volume divided by the air purge flow rate. Higher purge rates will decrease the front residence time for a fixed anode flow channel volume.
It has been observed that because the known anode purge occurs immediately at system shut-down when the fuel cell stack is still near its operating temperature, the air/hydrogen front still creates significant corrosion in the cathode catalyst. In other words, it has been discovered that temperature is an important factor in catalyst corrosion in that a lower stack temperature provides a lower stack degradation rate.